A variety of highly purified and partially purified lymphokines have been tested for their ability to inhibit the migration of human or guinea pig macrophages. These lymphokines have been obtained from normal human peripheral blood cells, from human T-T hybridomas and from human B-cell and T-cell lines. Three types of migration inhibitory factors (MIF) have been identified: T-cell derived, B-cell derived and interferon (IFN)-related classes. The relationships between these three types of MIF's are being investigated in this study by comparing their physiochemical characteristics, their modes of action, and their antigenic properties using monoclonal antibodies (MoAb). The relationships between MIF's and other lymphokine activities are also being investigated. MoAb against human alpha IFN, human gamma IFN and human IL-2, and antisera specific against human beta IFN, human CSF and human IL-1 are being used in the evaluations. Some cytokines, such as IL-1, IL-2, lymphotoxin and thymosin alpha-1 have been found to have no effect on macrophage migration. Alpha and gamma IFN have been shown to inhibit macrophage migration with gamma IFN having the most potent MIF-like activity per unit of antiviral activity (significant inhibition at 1-10 units/ml). Human gamma IFN produced in E. coli by recombinant-DNA technology also was found to have potent MIF-like activity indicating that the MIF activity of gamma IFN was not caused by a contaminant. This was confirmed by showing that the MoAb to gamma IFN neutralized all of the activity present in gamma IFN preparations. MIF activity has been obtained from the human B-cell line RPMI-1788. This activity was not neutralized by the anti-gamma IFN MoAb. However, 3 different partially purified MIF preparations having different molecular weights and PI's obtained from Harvard University were all neutralized by the anti-gamma IFN MoAb when tested on guinea pig macrophages. These observations have led us to postulate that T-cell derived MIF and gamma IFN may be structurally closely related and may actually represent 2 different lymphokine activities present within a single molecular type.